Builds.io is the web-site allowing you installation of iOS applications to your device directly from the browser (Mobile Safari). It contains applications thrown off from the official Apple AppStore because of violating Apple content policies.
The content policy of Builds.io is very loyal for indie developers and applied only one restriction: each application installable from Builds.io MUST NOT harm the device or device owner. So it's IMPOSSIBLE to install from Builds.io app containing viruses and malware or software engaged in espionage.
To install apps you should register first using this link: Sign up now. After registration process being complete, you should open activation link on the device you want install apps. It could be easily achieved using native Mail application on iOS device. After that you should accept mobile provision installing. Now, make payment for your device and proceed to Builds.io main page or to "Apps" link from within your user profile page. Tap "Install" button near the icon of interested application and you'll get the app to your device registered.